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Wednesday, January 9th, 2019
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Astronomers observe evolution of a black hole as it wolfs down stellar On March 11, an instrument aboard the International Space Station detected an enormous explosion of X-ray light that grew to be six times as bright as the Crab Nebula, nearly 10,000 light years away from Earth. Scientists determined the source was a black hole caught in the midst of an outburst — an extreme phase in which a black hole can spew brilliant bursts of X-ray energy as it devours an avalanche of gas and dust from a nearby star.
Now astronomers from MIT and elsewhere have detected “echoes” within this burst of X-ray emissions, that they believe could be a clue to how black holes evolve during an outburst. In a study published today in the journal Nature, the team reports evidence that as the black hole consumes enormous amounts of stellar material, its corona — the halo of highly-energized electrons that surrounds a black hole — significantly shrinks, from an initial expanse of about 100 kilometers (about the width of Massachusetts) to a mere 10 kilometers, in just over a month.
The findings are the first evidence that the corona shrinks as a black hole feeds, or accretes. The results also suggest that it is the corona that drives a black hole’s evolution during the most extreme phase of its outburst.
“This is the first time that we’ve seen this kind of evidence that it’s the corona shrinking during this particular phase of outburst evolution,” says Jack Steiner, a research scientist in MIT’s Kavli Institute for Astrophysics and Space Research. “The corona is still pretty mysterious, and we still have a loose understanding of what it is. But we now have evidence that the thing that’s evolving in the system is the structure of the corona itself.”
Steiner’s MIT co-authors include Ronald Remillard and first author Erin Kara.
X-ray echoes
The black hole detected on March 11 was named MAXI J1820+070, for the instrument that detected it. The Monitor of All-sky X-ray Image (MAXI) mission is a set of X-ray detectors installed in the Japanese Experiment Module of the International Space Station (ISS), that monitors the entire sky for X-ray outbursts and flares.
Soon after the instrument picked up the black hole’s outburst, Steiner and his colleagues started observing the event with NASA’s Neutron star Interior Composition Explorer, or NICER, another instrument aboard the ISS, which was designed partly by MIT, to measure the amount and timing of incoming X-ray photons.
“This boomingly bright black hole came on the scene, and it was almost completely unobscured, so we got a very pristine view of what was going on,” Steiner says.
A typical outburst can occur when a black hole sucks away enormous amounts of material from a nearby star. This material accumulates around the black hole, in a swirling vortex known as an accretion disk, which can span millions of miles across. Material in the disk that is closer to the center of the black hole spins faster, generating friction that heats up the disk.
“The gas in the center is millions of degrees in temperature,” Steiner says. “When you heat something that hot, it shines out as X-rays. This disk can undergo avalanches and pour its gas down onto the central black hole at about a Mount Everest’s worth of gas per second. And that’s when it goes into outburst, which usually lasts about a year.”
Scientists have previously observed that X-ray photons emitted by the accretion disk can ping-pong off high-energy electrons in a black hole’s corona. Steiner says some of these photons can scatter “out to infinity,” while others scatter back onto the accretion disk as higher-energy X-rays.
By using NICER, the team was able to collect extremely precise measurements of both the energy and timing of X-ray photons throughout the black hole’s outburst. Crucially, they picked up “echoes,” or lags between low-energy photons (those that may have initially been emitted by the accretion disk) and high-energy photons (the X-rays that likely had interacted with the corona’s electrons). Over the course of a month, the researchers observed that the length of these lags decreased significantly, indicating that the distance between the corona and the accretion disk was also shrinking. But was it the disk or the corona that was shifting in?
To answer this, the researchers measured a signature that astronomers know as the “iron line” — a feature that is emitted by the iron atoms in an accretion disk only when they are energized, such as by the reflection of X-ray photons off a corona’s electrons. Iron, therefore, can measure the inner boundary of an accretion disk.
When the researchers measured the iron line throughout the outburst, they found no measurable change, suggesting that the disk itself was not shifting in shape, but remaining relatively stable. Together with the evidence of a diminishing X-ray lag, they concluded that it must be the corona that was changing, and shrinking as a result of the black hole’s outburst.
“We see that the corona starts off as this bloated, 100-kilometer blob inside the inner accretion disk, then shrinks down to something like 10 kilometers, over about a month,” Steiner says. “This is the first unambiguous case of a corona shrinking while the disk is stable.”
“NICER has allowed us to measure light echoes closer to a stellar-mass black hole than ever before,” Kara adds. “Previously these light echoes off the inner accretion disk were only seen in supermassive black holes, which are millions to billions of solar masses and evolve over millions of years. Stellar black holes like J1820 have much lower masses and evolve much faster, so we can see changes play out on human time scales.”
While it’s unclear what is exactly causing the corona to contract, Steiner speculates that the cloud of high-energy electrons is being squeezed by the overwhelming pressure generated by the accretion disk’s in-falling avalanche of gas.
The findings offer new insights into an important phase of a black hole’s outburst, known as a transition from a hard to a soft state. Scientists have known that at some point early on in an outburst, a black hole shifts from a “hard” phase that is dominated by the corona’s energy, to a “soft” phase that is ruled more by the accretion disk’s emissions.
“This transition marks a fundamental change in a black hole’s mode of accretion,” Steiner says. “But we don’t know exactly what’s going on. How does a black hole transition from being dominated by a corona to its disk? Does the disk move in and take over, or does the corona change and dissipate in some way? This is something people have been trying to unravel for decades And now this is a definitive piece of work in regards to what’s happening in this transition phase, and that what’s changing is the corona.”
This research is supported, in part, by NASA through the NICER mission and the Astrophysics Explorers Program. | | 1:00p |
MIT adds computational Earth, atmospheric, and planetary sciences to its PhD offerings
The MIT Department of Earth, Atmospheric and Planetary Sciences (EAPS) has expanded its academic program to include a new doctoral field: computational Earth, atmospheric, and planetary sciences.
EAPS is now the latest department to participate in the Computational Science and Engineering (CSE) Program, which has been offering PhD degrees in computation since 2013. This move resonates with the Institute’s growing awareness of the advantages provided by education based in computation, as exemplified by the recent creation of the MIT Stephen A. Schwarzman College of Computing.
While enrolled in the CSE program, students are able to specialize at the doctoral level in a computation-related field of their choice through focused coursework and a doctoral thesis through a number of participating host departments, including Aeronautics and Astronautics, Chemical Engineering, Civil and Environmental Engineering, Mechanical Engineering, Mathematics, Nuclear Science and Engineering, and now EAPS.
EAPS-affiliated CSE graduate students will be able to analyze complex Earth and planetary systems, and mysteries of the natural world, leveraging cutting-edge computing and data science.
“Computation is playing an ever-growing role in addressing earth science questions, like those central to the study of climate and earthquakes,” says Raffaele Ferrari, the Cecil and Ida Green Professor of Oceanography and chair of the EAPS Program in Atmospheres, Oceans and Climate (PAOC), who helped to shepherd the creation of the new degree. “Adding the PhD track will ensure that the department remains an educational leader, fostering the next generation of earth researchers.”
The new computational Earth, atmospheric, and planetary sciences degree offers several advantages for the department and its graduate students: EAPS members are already performing state-of-the-art computational research, and Earth science disciplines are increasingly incorporating elements of artificial intelligence and machine learning. By integrating modern computational techniques and machine intelligence developed at MIT into the curriculum, the department will improve the visibility of this research, enhance its ability to attract and produce top talent in computational science, as well as remain at the forefront of geoscience study and education.
MIT has progressively invested in computation’s use around the Institute and in the classroom, with several initiatives and the formation of the MIT Schwarzman College of Computing. Sharing MIT’s vision, thesis activities of EAPS computational Earth, atmospheric, and planetary sciences candidates will emphasize the development of new computational methods and/or the innovative application of computational techniques to important problems in fields ranging from geophysics to climate science.
Finally, this interdisciplinary training will equip EAPS faculty and students to tackle next-level research questions in novel ways. Having received credit for their work, EAPS graduates of the program will come away with enhanced opportunities for when they take their next career step.
"This degree will offer graduate students the skills to tackle computationally demanding problems related to the Earth and planets," says Taylor Perron, EAPS associate professor of geology and associate department head, who helps to oversee the department’s educational program, "and the opportunity to interact with other computation and data specialists from across the Institute."
| | 3:00p |
Revolutionary radio telescope detects bevy of fast radio bursts Scientists at the MIT Kavli Institute for Astrophysics and Space Research are part of a team that has discovered 13 fast radio bursts (FRBs), as well as the second repeating FRB ever recorded, using a revolutionary radio telescope.
FRBs are short flashes of radio waves coming from far outside our Milky Way galaxy. Scientists believe FRBs emanate from powerful astrophysical phenomena billions of light years away, but they have yet to determine their origin.
These discoveries are among the first, eagerly awaited results from the Canadian Hydrogen Intensity Mapping Experiment (CHIME), a revolutionary radio telescope inaugurated in late 2017 by a collaboration of scientists that includes MIT’s Kiyoshi Masui, an assistant professor of physics, and Juan Mena Parra, a Kavli postdoc.
Masui and Mena Parra joined the MIT School of Science last fall from the University of British Columbia and McGill University respectively, where they worked on the Canadian-led project for the past five years.
In two papers published today in Nature and presented the same day at the American Astronomical Society meeting in Seattle, the researchers present data from the 13 bursts while CHIME was in its precommissioning phase and running at only a fraction of its full capacity.
A new way of building radio telescopes
In 2007, astronomers discovered a new phenomenon that they called an FRB. Data recorded several years earlier by the Parkes Radio Telescope in Australia showed a fleeting but powerful radio emission coming from an unidentified source in space.
Since the first discovery a decade earlier, roughly 60 bursts have been observed by five different telescopes worldwide. In stark contrast, the data presented today from 13 bursts was collected over a period of only three weeks during the summer of 2018. The scientists also discovered repeat bursts from one of the 13 sources, a discovery only made once before.
The stunning rate at which CHIME detected the FRBs is due to its revolutionary design.
“The telescope has no moving parts. Instead it uses digital signal processing to ‘point’ the telescope and reconstruct where the radio waves are coming from,” says Masui. “This is done using clever algorithms and a couple of giant computer clusters that sit beside the telescope and crunch away at the data in real time.”
Physics professor Max Tegmark pioneered these kinds of telescopes. He led a study in 2009 that worked out the details of how such a telescope would work. He also led the construction of a prototype telescope, MITEoR, that tested algorithms and calibration techniques. With CHIME, this concept for telescope design comes of age and is being used for breakthrough science to detect FRBs.
Discovery of second repeating FRB suggests more exist
Of the FRBs observed to date, repeating bursts from a single source had been found only once before — a discovery made by the Arecibo radio telescope in Puerto Rico in 2015.
“Until now, there was only one known repeating FRB. Knowing that there is another suggests that there could be more out there. And with more repeaters available for study, we may be able to understand these cosmic puzzles a bit better — where they’re from, what causes them, and why,” says Ingrid Stairs, a member of the CHIME team and an astrophysicist at University of British Columbia.
Before CHIME began to gather data, some scientists wondered if the range of radio frequencies the telescope had been designed to detect would be too low to pick up fast radio bursts. Most of the FRBs previously detected had been found at frequencies near 1,400 megahertz, well above the Canadian telescope’s range of 400 megahertz to 800 megahertz.
The CHIME team’s recent results settles these doubts, with the majority of the 13 bursts being recorded well down to the lowest frequencies in CHIME’s range. In some of the 13 cases, the signal at the lower end of the band was so bright that it seems likely other FRBs will be detected at frequencies even lower than CHIME’s minimum of 400 megahertz.
FRB sources likely to be in "special places" within galaxies
Ever since FRBs were first detected, scientists have been piecing together the signals’ observed characteristics to come up with models that might explain the sources of the mysterious bursts and provide some idea of the environments in which they occur. The detection by CHIME of FRBs at lower frequencies means some of these theories will need to be reconsidered.
The majority of the 13 FRBs detected showed signs of “scattering,” a phenomenon that reveals information about the environment surrounding a source of radio waves. CHIME measures scattering more precisely than other instruments because it operates at lower frequencies. The amount of scattering observed by the CHIME team led them to conclude that the sources of FRBs are powerful astrophysical objects more likely to be in locations with special characteristics.
“That could mean in some sort of dense clump like a supernova remnant,” says team member Cherry Ng, an astronomer at the University of Toronto. “Or near the central black hole in a galaxy. But it has to be in some special place to give us all the scattering that we see.”
CHIME is a collaboration of more than 50 scientists led by the University of British Columbia, McGill University, University of Toronto, and the National Research Council of Canada (NRC). The $16-million investment for CHIME was provided by the Canada Foundation for Innovation and the governments of British Columbia, Ontario and Quebec, with additional funding from the Dunlap Institute, the Natural Sciences and Engineering Research Council and the Canadian Institute for Advanced Research. The telescope is located in the mountains of British Columbia’s Okanagan Valley at the NRC’s Dominion Radio Astrophysical Observatory near Penticton.
This article is adapated from a press release issued by McGill University. | | 5:14p |
X-ray pulse detected near event horizon as black hole devours star On Nov. 22, 2014, astronomers spotted a rare event in the night sky: A supermassive black hole at the center of a galaxy, nearly 300 million light-years from Earth, ripping apart a passing star. The event, known as a tidal disruption flare, for the black hole’s massive tidal pull that tears a star apart, created a burst of X-ray activity near the center of the galaxy. Since then, a host of observatories have trained their sights on the event, in hopes of learning more about how black holes feed.
Now researchers at MIT and elsewhere have pored through data from multiple telescopes’ observations of the event, and discovered a curiously intense, stable, and periodic pulse, or signal, of X-rays, across all datasets. The signal appears to emanate from an area very close to the black hole’s event horizon — the point beyond which material is swallowed inescapably by the black hole. The signal appears to periodically brighten and fade every 131 seconds, and persists over at least 450 days.
The researchers believe that whatever is emitting the periodic signal must be orbiting the black hole, just outside the event horizon, near the Innermost Stable Circular Orbit, or ISCO — the smallest orbit in which a particle can safely travel around a black hole.
Given the signal’s stable proximity to the black hole, and the black hole’s mass, which researchers previously estimated to be about 1 million times that of the sun, the team has calculated that the black hole is spinning at about 50 percent the speed of light.
The findings, reported today in the journal Science, are the first demonstration of a tidal disruption flare being used to estimate a black hole’s spin.
The study’s first author, Dheeraj Pasham, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research, says that most supermassive black holes are dormant and don’t usually emit much in the way of X-ray radiation. Only occasionally will they release a burst of activity, such as when stars get close enough for black holes to devour them. Now he says that, given the team’s results, such tidal disruption flares can be used to estimate the spin of supermassive black holes — a characteristic that has been, up until now, incredibly tricky to pin down.
“Events where black holes shred stars that come too close to them could help us map out the spins of several supermassive black holes that are dormant and otherwise hidden at the centers of galaxies,” Pasham says. “This could ultimately help us understand how galaxies evolved over cosmic time.”
Pasham’s co-authors include Ronald Remillard, Jeroen Homan, Deepto Chakrabarty, Frederick Baganoff, and James Steiner of MIT; Alessia Franchini at the University of Nevada; Chris Fragile of the College of Charleston; Nicholas Stone of Columbia University; Eric Coughlin of the University of California at Berkeley; and Nishanth Pasham, of Sunnyvale, California.
A real signal
Theoretical models of tidal disruption flares show that when a black hole shreds a star apart, some of that star's material may stay outside the event horizon, circling, at least temporarily, in a stable orbit such as the ISCO, and giving off periodic flashes of X-rays before ultimately being fed by the black hole. The periodicity of the X-ray flashes thus encodes key information about the size of the ISCO, which itself is dictated by how fast the black hole is spinning.
Pasham and his colleagues thought that if they could see such regular flashes very close to a black hole that had undergone a recent tidal disruption event, these signals could give them an idea of how fast the black hole was spinning.
They focused their search on ASASSN-14li, the tidal disruption event that astronomers identified in November 2014, using the ground-based All-Sky Automated Survey for SuperNovae (ASASSN).
“This system is exciting because we think it’s a poster child for tidal disruption flares,” Pasham says. “This particular event seems to match many of the theoretical predictions.”
The team looked through archived datasets from three observatories that collected X-ray measurements of the event since its discovery: the European Space Agency’s XMM-Newton space observatory, and NASA’s space-based Chandra and Swift observatories. Pasham previously developed a computer code to detect periodic patterns in astrophysical data, though not for tidal disruption events specifically. He decided to apply his code to the three datasets for ASASSN-14li, to see if any common periodic patterns would rise to the surface.
What he observed was a surprisingly strong, stable, and periodic burst of X-ray radiation that appeared to come from very close to the edge of the black hole. The signal pulsed every 131 seconds, over 450 days, and was extremely intense — about 40 percent above the black hole’s average X-ray brightness.
“At first I didn’t believe it because the signal was so strong,” Pasham says. “But we saw it in all three telescopes. So in the end, the signal was real.”
Based on the properties of the signal, and the mass and size of the black hole, the team estimated that the black hole is spinning at least at 50 percent the speed of light.
“That’s not super fast — there are other black holes with spins estimated to be near 99 percent the speed of light,” Pasham says. “But this is the first time we’re able to use tidal disruption flares to constrain the spins of supermassive black holes.”
Illuminating the invisible
Once Pasham discovered the periodic signal, it was up to the theorists on the team to find an explanation for what may have generated it. The team came up with various scenarios, but the one that seems the most likely to generate such a strong, regular X-ray flare involves not just a black hole shredding a passing star, but also a smaller type of star, known as a white dwarf, orbiting close to the black hole.
Such a white dwarf may have been circling the supermassive black hole, at ISCO — the innermost stable circular orbit — for some time. Alone, it would not have been enough to emit any sort of detectable radiation. For all intents and purposes, the white dwarf would have been invisible to telescopes as it circled the relatively inactive, spinning black hole.
Sometime around Nov. 22, 2014, a second star passed close enough to the system that the black hole tore it apart in a tidal disruption flare that emitted an enormous amount of X-ray radiation, in the form of hot, shredded stellar material. As the black hole pulled this material inward, some of the stellar debris fell into the black hole, while some remained just outside, in the innermost stable orbit — the very same orbit in which the white dwarf circled. As the white dwarf came in contact with this hot stellar material, it likely dragged it along as a luminous overcoat of sorts, illuminating the white dwarf in an intense amount of X-rays each time it circled the black hole, every 131 seconds.
The scientists admit that such a scenario would be incredibly rare and would only last for several hundred years at most — a blink of an eye in cosmic scales. The chances of detecting such a scenario would be exceedingly slim.
“The problem with this scenario is that, if you have a black hole with a mass that’s 1 million times that of the sun, and a white dwarf is circling it, then at some point over just a few hundred years, the white dwarf will plunge into the black hole,” Pasham says. “We would’ve been extremely lucky to find such a system. But at least in terms of the properties of the system, this scenario seems to work.”
The results’ overarching significance is that they show it is possible to constrain the spin of a black hole, from tidal disruption events, according to Pasham. Going forward, he hopes to identify similar stable patterns in other star-shredding events, from black holes that reside further back in space and time.
“In the next decade, we hope to detect more of these events,” Pasham says. “Estimating spins of several black holes from the beginning of time to now would be valuable in terms of estimating whether there is a relationship between the spin and the age of black holes.”
This research was supported, in part, by NASA. |
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